1. Field of the invention
The present invention generally relates to a digital demodulation technique in a wireless communication system.
2. Description of the Related Art
In a digital demodulator, coherent detection or differential detection is performed before bit synchronization and then symbol decision is performed using a detected baseband signal at bit-synchronized sampling points. However, according to such a demodulation method, the carrier recovery and symbol synchronization performance have a great effect on the receiving characteristic.
It is also known that the demodulation performance is improved by receiving a predetermined training signal or a preamble signal. More specifically, the predetermined training signal is used to calculate a decision error. The decision error is fed back to an equalizer and a phase-locked loop to compensate for distortions including linear distortion and group delay distortion.
In the case of mobile communications, however, since radio channel characteristics frequently vary, it is necessary to transmit the training signal for each burst transmission to compensate for variations in radio propagation characteristic. Therefore, the overhead becomes larger and thereby the throughput of the communications system is reduced. Contrarily, as the training bit length becomes shorter, the amount of transmission data is increased but the synchronization error is also increased. Similarly, in packet communications, the shorter the sync-bit length in a packet, the larger the symbol synchronization error.